The present invention relates to a field emission-type electron source arranged to emit an electron beam by using a semiconductor material according to field emission.
Conventionally, as a field emission-type electron source (which hereinbelow may be shortly referred to as an xe2x80x9celectron sourcexe2x80x9d), there is known a Spindt-type electrode disclosed in, for example, U.S. Pat. No. 3,665,241. The Spindt-type electrode includes a substrate and gate layers, in which a large number of trigonal-pyramid-shaped emitter chips are arranged on the substrate, and the gate layers are insulated from one-way emitter chips that have emission openings provided to expose end portions of the emitter chips. In the Spindt-type electrode, when a high voltage is applied in a vacuum to cause the emitter chips to be cathodic with respect to the gate layer, electron beams are emitted from the apexes of the emitter chips through the emission openings.
For the Spindt-type electrode, however, a manufacturing process is complicated, and it is difficult to manufacture a large number of the trigonal-pyramid-shaped emitter chips at high precision. As such, a problem arises in that it is difficult to implement area enlargement when the electrode is used for, for example, a flat emitting device and display. Moreover, in the Spindt-type electrode, since fields concentrate at the apex of the emitter chip, when a residual gas exists because the degree of a vacuum around the apex of the emitter chip is low, the residual gas is ionized by emitted electrons to be of anodic ion. Since the anodic ions impinge on the apex of the emitter chip, the apex of the emitter chip suffers damage (such as ion-impact-caused damage). For this reason, defects can easily occur to an extent that electron properties, such as the current density and emission efficiency, become unstable, and hence the service life of the emitter chip is reduced. To prevent the defects, the Spindt-type electrode needs to be used in a high vacuum (in a range of 10xe2x88x925 Pa to 10xe2x88x926 Pa). This arises problems, however, in that costs are increased, and in addition, handling becomes difficult.
For eliminating the defects described above to implement improvement, an electron source of a MIM (metal insulator metal) type and an electron source of a MOS (metal oxide semiconductor) type have been proposed. The former is a flat electron source that has a (metal)-(insulator film)-(metal) multilayered structure, and the latter is a flat electron source that has a (metal)-(oxide)-(film semiconductor) multilayered structure. To improve the emission efficiency (to cause many electrons to emit) in either of the electron sources of the aforementioned types, the film thickness of the film such as the insulator film or the oxygen film needs to be reduced. However, with the insulator film or the oxygen film of which the thickness is excessively reduced, when voltage is applied between upper and lower electrodes in the multilayered structure, dielectric breakdown can occur. Since the electrical breakdown needs to be prevented, the reduction in the insulating film or the oxygen film is limited. As such, a problem arises in that the electron emission efficiency (induction efficiency) cannot be increased so high.
Recently, as is disclosed in Japanese Unexamined Patent Application Publication No. 8-250766, an electron source (cold electron emission semiconductor device) has been proposed. The electron source is configured such that a monocrystalline semiconductor substrate such as a silicon substrate is used, a surface of the semiconductor substrate is anodic-oxidized, a porous semiconductor layer (porous silicon layer) is thereby formed, and a thin metal film is formed on the porous semiconductor layer. In the electron source, voltage is applied between the semiconductor substrate and the thin metal film to cause electrons to emit.
However, in the electron source proposed in Japanese Unexamined Patent Application Publication No. 8-250766, since the substrate is limited to be of a semiconductor, a problem arises in that it is difficult to implement the area enlargement and the cost reduction. In addition, a so-called popping phenomenon tends to occur in electron emission, and hence nonuniformity tends to occur in light emission. As such, in a state where the electron source is used with, for example, a flat emitting device or display, light-emission nonuniformity can occur.
In view of the above, with Japanese Patent Applications No. 10-272340 and No. 10-272342, the inventors proposed an electron source configured such that, a porous polycrystalline semiconductor layer (such as a porous polycrystalline silicon layer) is interposed between an electroconductive substrate and a thin metal film (surface electrode) by performing, for example, rapid thermal oxidation at 900xc2x0 C. according to a rapid thermal oxidation (RTO) technique; and thereby, a strong field drift layer (which hereinbelow will be referred to as a xe2x80x9cdrift layerxe2x80x9d) in which electrons injected from the electroconductive substrate drift is formed.
As shown in FIG. 43, in an electron source 10xe2x80x2 of the aforementioned type, a drift layer 6 is formed on a main surface of an n-type silicon substrate 1, which is an electroconductive substrate, in which the drift layer 6 is formed of an oxidized porous polycrystalline silicon layer. A surface electrode 7 made of a thin metal film is formed on the drift layer 6. An ohmic electrode 2 is formed on a reverse surface of the n-type silicon substrate 1. The thickness of the drift layer 6 is, for example, 1.5 xcexcm.
As shown in FIG. 44, in the electron source 10xe2x80x2, the surface electrode 7 is disposed to be exposed to a vacuum. A collector electrode 12 is disposed to oppose the surface electrode 7. In the configuration, a direct-current voltage Vps is applied to cause the surface electrode 7 to be anodic with respect to the n-type silicon substrate 1 (ohmic electrode 2). In addition, a direct-current voltage Vc is applied to cause the collector electrode 12 to be anodic with respect to the surface electrode 7. Thereby, electrons injected from the n-type silicon substrate 1 into the drift layer 6 are caused to drift, and are discharged through the surface electrode 7 (each of single-dotted chain lines in FIG. 44 shows the flow of an electron exe2x88x92 emitted through the surface electrode 7). As such, a material having a small work function is preferably used for the surface electrode 7. A current flowing between the surface electrode 7 and the ohmic electrode 2 is generally called a diode current Ips, and a current flowing between the collector electrode 12 and the surface electrode 7 is generally called an emitted electron current Ie. The greater the emitted electron current Ie with respect to the diode current Ips (Ie/Ips), the higher the electron emission efficiency. In the electron source 10xe2x80x2, electrons can be emitted even when the direct-current voltage Vps to be applied between the surface electrode 7 and the ohmic electrode 2 is in a low range of from 10 to 20 V.
The electron source 10xe2x80x2 enables electrons to be stably emitted at high electron emission efficiency without causing popping phenomenon since it has a less dependency to the degree of vacuum as an electron emission property.
As shown in FIG. 45, the drift layer 6 includes at least grain 51 (semiconductor crystal) made of columner polycrystalline silicon disposed on the main surface of the n-type silicon substrate 1; a thin silicon oxide film 52 formed on a surface of the grain 51; fine silicon crystal 63 on the order of nanometer that is interposed between items of the grain 51; and a silicon oxide film 64 provided as an insulator film that is formed on a surface of the fine silicon crystal 63 and that has a thickness smaller than a crystal grain diameter of the fine silicon crystal 63. That is, in the drift layer 6, the surface of each item of the grain 51 becomes porous, and a crystal condition is maintained in a central portion of each item of the grain 51. As such, most part of the field applied to the drift layer 6 is exerted on the silicon oxide film 64. Hence, the injected electrons are accelerated by a strong field applied on the silicon oxide film 64, and is caused thereby to drift between items of the grain 51 toward the surface as shown by an arrow A. Thereby, the electron emission efficiency can be improved. Electrons reached the surface of the drift layer 6 are hot electrons that easily pass through the surface electrode 7 and that are emitted in a vacuum. The film thickness of the surface electrode 7 is set in a range of from 10 to 15 nm.
Instead of the semiconductor substrate such as the n-type silicon substrate 1 as the electroconductive substrate, a substrate formed such that a lower electrode made of a conductive layer (such as a thin metal film) is formed on a dielectric substrate such as a glass substrate can be used. Thereby, further electron-source area enlargement and cost reduction can be implemented.
FIG. 46 shows an electron source 10xe2x80x3 using an electroconductive substrate formed of a dielectric substrate 11 made of a glass substrate, and a lower electrode 8 formed on a main surface of the dielectric substrate 11. As shown in FIG. 46, in the electron source 10xe2x80x3, the lower electrode 8 made of the conductive layer is formed on the main surface of the dielectric substrate 11. A drift layer 6 is formed on the lower electrode 8. A surface electrode 7 made of a thin metal film is formed on the drift layer 6. The drift layer 6 is formed such that, after an undoped polycrystalline silicon layer is overlaid on the lower electrode 8, the polycrystalline silicon layer is processed porous according to an anodic oxidation treatment, and is then oxidized or nitrided according to, for example, a rapid thermal technique performed at 900xc2x0 C.
As shown in FIG. 47, in substantially the same manner as that in the electron source 10xe2x80x2 (refer to FIG. 44), in the electron source 10xe2x80x3, a surface electrode 7 is disposed to be exposed to a vacuum, and a collector electrode 12 is disposed to oppose the surface electrode 7. In addition, in substantially the same way as in the electron source 10xe2x80x2, direct-current voltages Vps and Vc are applied, electrons injected from the lower electrode 8 into the drift layer 6 are caused to drift in the drift layer 6, and are emitted through the surface electrode 7. Also in the electron source 10xe2x80x3, electrons can be emitted even when the direct-current voltage Vps, which is applied between the surface electrode 7 and the lower electrode 8, is in a low range of from 10 to 20 V.
As shown in FIG. 48, the electron source 10xe2x80x3 can be used as a display-dedicated electron source. In the display shown in FIG. 48, a glass substrate 14 is disposed to oppose the electron source 10xe2x80x3. On a surface of the glass substrate 14, which opposes the electron source 10xe2x80x3, a collector electrode 12 and a phosphor layer 15 are provided. The phosphor layer 15 is coated on a surface of the collector electrode 12 to emit visible light according to electrons emitted from the electron source 10xe2x80x3. The glass substrate 14 is spaced with a spacer (not shown) from the electron source 10xe2x80x3. A hermetic space formed between the glass substrate 14 and the electron source 10xe2x80x3 is set to a vacuum state.
The electron source 10xe2x80x3 used in the display shown in FIG. 48 includes a dielectric substrate 11 made of a glass substrate; a plurality of lower electrodes 8 arranged on a main surface of the dielectric substrate 11; a drift layer 6 including a plurality of drift portions 6a individually made of an oxidized porous polycrystalline silicon layer in such a manner as to overlap the lower electrodes 8, and a plurality of isolating portions 6b that are formed of a polycrystalline silicon layer and that individually fill up spaces between the drift portions 6a; and a plurality of surface electrodes 7 arranged on the drift layer 6 in such a manner as to cross the drift portions 6a and the isolating portions 6b in the direction intersecting with the lower electrodes 8.
In the electron source 10xe2x80x3, the drift portions 6a of the drift layer 6 are sandwiched between the plurality of lower electrodes 8 and the plurality of surface electrodes 7. As such, when an associated set of the surface electrode 7 and the lower electrode 8 is desirably selected, and voltage is applied into the selected couple, a strong field is applied to the drift portion 6a in the position corresponding to the intersection of the selected surface electrode 7 and lower electrode 8, and electrons are thereby emitted. Specifically, as in a case where electron sources are individually disposed at intersections in a check pattern formed of the surface electrodes 7 and the lower electrodes 8, an associated set of the surface electrode 7 and the lower electrode 8 is selected, and thereby, electrons can be emitted from a desired intersection. The voltage to be applied between the surface electrode 7 and the lower electrode 8 is set to a range of from 10 to 20 V.
In the electron source 10xe2x80x3 used in the display shown in FIG. 48, an undoped polycrystalline silicon layer is made porous in depth up to a portion reaching the lower electrode 8.
However, as shown in FIG. 49, the polycrystalline silicon layer may be made porous in depth up to a portion not reaching the lower electrode 8. In this case, an undoped polycrystalline silicon layer 3 is interposed between the lower electrode 8 and the drift portion 6a. 
The electron source 10xe2x80x3 used in the display shown in FIG. 48 has a so-called a passive matrix structure in which the surface electrodes 7 and the lower electrodes 8 mutually opposes in a matrix so as to sandwich the drift layer 6.
As shown in FIG. 50, the drift portions 6a are assumed to be resistors R. In this case, among the plurality of surface electrodes 7, those selected are assumed to be set to an H level, and those unselected are assumed to be set to an L level. On the other hand, among the plurality of lower electrodes 8, those selected are assumed to be set to an L level, and those unselected are assumed to be set to an H level. In this case, as shown by a single-dotted chain line in FIG. 50, a current I1 is led to flow through a passageway (surface electrode 7 at the H level)-(resistor R)-(lower electrode 8 at the L level). However, in the electron source in which the drift portions 6a are made of resistors R, many passageways exist that pass leakage current flowing reversely to the lower electrodes 8 at the H level to the surface electrodes 7 at the L level. As such, the current flows even to the unselected intersections in the check pattern, thereby increasing the power consumption.
However, resistors are not used for the drift portions 6a in the electron source 10xe2x80x3 shown in FIG. 48, which has the passive matrix structure. In the electron source 10xe2x80x3, the overlapped portions of the surface electrodes 7 and the drift portions 6a are individually used as electron sources. Suppose the direction along which current flows from the surface electrode 7 to the lower electrode 8 in each of the electron sources is a forward direction. In this case, a current-voltage property is of a nonlinear type between the surface electrode 7 and the lower electrode 8 in each of the electron sources. Accordingly, leakage current is reduced less than that in the case where the drift portion 6a is assumed to be the resistor R. However, to implement the area enlargement of the electron source 10xe2x80x3, the total amount of leakage current cannot be neglected. Thus, problems arise in that reduction in power consumption and improvement in electron emission efficiency are hindered.
As shown in FIG. 51, the above-described leakage-current flow can be prevented if the configuration is made such that a diode D having the anode on the side of the surface electrode 7 and the cathode on the side of the lower electrode 8 is formed between each of the surface electrodes 7 and lower electrodes 8. However, the diode D is not formed between the surface electrode 7 and the lower electrode 8 in each of the electron sources of the electron source 10xe2x80x3 shown in FIG. 48. This arises problems in that, as shown by a double-dotted chain line in FIG. 51, leakage current flows from the lower electrode 8 at the H level to the surface electrode 7 at the L level, thereby making it difficult to reduce the power consumption and to improve the electron emission efficiency.
FIG. 56 is a graph representing the relationship between voltage and current in cases where a forward voltage and a backward voltage are applied. As shown in FIG. 56, even in the conventional electron source, a rectification property can be obtained to a certain extent in the forward current and the backward current. However, with the rectification property at the illustrated levels, it is still difficult to sufficiently minimize the leakage current.
As a means to solve these problems, it can be considered that the electron source is configured as shown in FIG. 52. Specifically, an n-type polycrystalline silicon region 41 is formed on the side of a surface of an undoped polycrystalline silicon layer 3 to be spaced away from a drift portion 6a. In addition, a p-type polycrystalline silicon region 42 is formed on the side of a surface in the n-type polycrystalline silicon region 41. A surface electrode 7 is formed in such a manner as to cross the drift portion 6a and a part of the n-type polycrystalline silicon region 41. In addition, a dummy surface electrode 17 is provided on the p-type polycrystalline silicon region 42 to add a rectification property to the current-voltage property in a portion between the dummy surface electrode 17 and the lower electrode 8.
In the electron source shown in FIG. 52, however, the n-type polycrystalline silicon region 41 and the p-type polycrystalline silicon region 42 need to be provided to be spaced away from the drift portion 6a; and in addition, the dummy surface electrode 17 needs to be provided to be spaced away from the surface electrode 7. There arise problems in that a per-unit-area electron emitting area is reduced with a passive matrix structure being employed.
In addition, in the electron source 10xe2x80x3 shown in FIG. 48, in which patterning is performed for the drift portions 6a are patterned, the field intensity of a portion of the drift portion 6a in the vicinity of a boundary to the isolating portion 6b is higher than the field intensity of a central portion of the drift portion 6a. Accordingly, the per-unit-area electron emission amount in the aforementioned vicinity of the boundary is greater than the per-unit-area electron emission amount in the central portion of the drift portion 6a. This arises a problem in that electrons are excessively emitted through the aforementioned vicinity of the boundary.
Moreover, since the field intensity in the aforementioned vicinity of the boundary high, a case can occur in which dielectric breakdown occurs in the drift portion 6a (the drift portion 6a deteriorates) in the aforementioned vicinity of the boundary, and hence excessive current locally flows between the lower electrode 8 and the surface electrode 7. Because of the flow of excessive current, problems are caused in that local heating occurs in the surface electrode 7, which is formed of the electroconductive thin film, and/or the lower electrode 8 (conductive layer); and hence the level of deterioration in, for example, the surface electrode 7 and the drift portion 6a is increased. The field intensity in the aforementioned vicinity of the boundary becomes higher than the field intensity in the central portion of the drift portion 6a for the reason that the porosity or the extent of oxidation or nitrization is different in the central portion and the aforementioned of the drift layer 6.
In the electron source 10xe2x80x2 or 10xe2x80x3 shown in FIG. 43 or 46, electron emission properties thereof include a less dependency to the degree of vacuum, no popping phenomenon occurs in electron emission, and electrons can be stably emitted at high electron emission efficiency. Nevertheless, however, in the electron source 10xe2x80x2 or 10xe2x80x3, the diode current Ips gradually varies as time passes as shown by a graph P in FIG. 53, and the emitted electron current Ie gradually varies as time passes as shown by a graph Q in the same figure. Specifically, since the diode current Ips gradually increases, and the emitted electron current Ie gradually decreases, the electron emission efficiency gradually decreases. In this case, efforts can be exerted to inhibit the gradual reduction; however, it involves the problem of increasing the power consumption.
These problems are considered to occur for the following reasons. In the electron source 10xe2x80x2 or 10xe2x80x3, since the drift layer 6 is formed according to the oxidation of the porous polycrystalline silicon layer, it is difficult to form the silicon oxide films 52 and 64 (refer to FIG. 45) uniformed in quality and thicknesses for the entirety of the drift layer 6. In addition, in the drift layer 6, in comparison between the total film thickness of the silicon oxide films 64 in the region where the fine silicon crystal 63 is formed and the thickness of the silicon oxide film 52 in a portion where the grain 51 remains, the silicon oxide film 52 tends to be thinner. As such, when a driving voltage (direct-current voltage Vps) is applied to the electron source 10xe2x80x2 or 10xe2x80x3, and the diode current Ips is thereby applied to flow therethrough, dielectric breakdown gradually occurs in, for example, portions where the film thicknesses are insufficient, defective portions, and portions including a large amount of impurity in the silicon oxide film 52 or the silicon oxide film 64 or both the silicon oxide film 52 and silicon oxide film 64. In a portion where dielectric breakdown has occurred, the resistance values of the silicon oxide films 52 and 64 are reduced, whereas the diode current Ips gradually increases. On the other hand, current contributing to electron emission decreases, and the emitted electron current Ie gradually decreases.
For the above reasons, when the electron source 10xe2x80x2 or 10xe2x80x3 is used with, for example, a display, because of dielectric breakdown occurring in the silicon oxide films 52 and 64, problems are caused in that the power consumption and the heating value are gradually increase, thereby causing the luminance to gradually decrease.
The electron source 10xe2x80x3 shown in FIG. 46 or 49 can be used as a display-dedicated electron source shown in FIG. 54. The electron source 10xe2x80x3 shown in FIG. 54 includes a dielectric substrate 11 made of a glass substrate; a plurality of wirings 8a (lower electrodes 8) arranged on a main surface of the dielectric substrate 11; a drift layer 6 including a plurality of drift portions 6a formed of an oxidized porous polycrystalline silicon layer in such a manner as to overlap the wirings 8a, and isolating portions 6b that are formed of a polycrystalline silicon layer and that individually fill up spaces between the drift portions 6a; a plurality of surface electrodes 7 that individually oppose the wirings 8a via the drift portions 6a; and a plurality of bus electrodes 25 commonly coupling the plurality of surface electrode 7, which are arranged in the direction intersecting with the wirings 8a, in units of each row on the drift layer 6. The bus electrodes 25 are arranged in such a manner as to cross the drift portions 6a and the isolating portions 6b in the direction intersecting with the wirings 8a. 
In substantially the same manner as that in the electron source 10xe2x80x3 shown in FIG. 48, in the electron source 10xe2x80x3, when an associated set of the bus electrode 25 and the wiring 8a is selected, electrons can be emitted from a desired intersection. The wiring 8a is formed as a stripe having two end portions in a longitudinal direction on which pads 27 are individually formed. The bus electrode 25 is connected to pads 28 through the individual pads 27.
However, in the electron source 10xe2x80x3 shown in FIG. 54, when an overcurrent flows between the bus electrode 25 and the surface electrode 7, for example, cases can occur in which electrons excessively are emitted from the drift portion 6a corresponding to the selected intersection, and dielectric breakdown occurs with the intersection corresponding to the selected intersection, thereby causing a short-circuit current to flow between the wiring 8a and the surface electrode 7. This arises problems in that the temperature increases in the drift portion 6a, and the surface electrode 7, and the wiring 8a; and the deterioration continues for the overall electron source, thereby reducing the reliability thereof. That is, problems occur in that deterioration is introduced not only to the drift portion 6a, the surface electrode 7, and/or the wiring 8a that correspond to the selected intersection, but also to the drift portion 6a, the surface electrode 7, and/or the wiring 8a that correspond to an unselected intersection. In addition, since excessive electrons are emitted from the drift portion 6a that caused the dielectric breakdown, when the electron source is used with a display, the luminance of a specific pixel abnormally increases, intrascreen nonuniformity in luminance increases.
The electron source 10xe2x80x3 or the display, which is shown in FIG. 54, includes a faceplate that is made of a glass substrate and that is disposed opposite to the electron source 10xe2x80x3.
As shown in FIG. 55, pixels 31 are provided in units of the individual surface electrodes 7 of the electron source 10xe2x80x3. Three phosphor cells 32a, 32b, and 32c corresponding to the three primitive colors of R, G, and B are coated and formed in each of the pixels 31. The individual pixels 31 and the phosphor cells 32a, 32b, and 32c in each of the pixels 31 are individually isolated by isolating layers 33 formed of a black pattern called a black stripe.
In substantially the same manner as that in the electron source 10xe2x80x3 shown in FIG. 48, in the electron source 10xe2x80x3 shown in FIG. 54, the per-unit-area electron emission amount in the vicinity of a boundary to the isolating portion 6b is greater than the per-unit-area electron emission amount in a central portion of the drift portion 6a in the direction along which the wiring 8a extends; hence electrons are excessively emitted through the aforementioned vicinity of the boundary. As such, when the configuration is made such that the distance between the pixels 31 is reduced, and the size (area) of the pixel 31 is reduced, bleeding occurs in the individual pixels 31. This makes it difficult to implement high precision display.
The present invention is made to solve the above-described problems, and an object thereof is to provide an electron source (field emission-type electron source) that enables a power-consumption reduction to be implemented without reducing a per-unit-area field emission area in comparison to the conventional cases. Another object is to provide an electron source that enables the prevention of emission of excessive electrons Still another object is to provide an electron source that has a high ageing stability as an electron emission property. Yet another object is to provide a high-reliability electron source that can be used as an electron source in a high-precision display.
An electron source (field emission-type electron source) is characterized by including a substrate, an electroconductive layer formed on a surface of the substrate, a semiconductor layer formed on the electroconductive layer, a strong field drift layer including a drift portion that is made of an oxidized or nitrided porous semiconductor layer and that is formed on the side of the surface of the semiconductor layer, and a surface electrode formed on the strong field drift layer, wherein when voltage is applied to cause the surface electrode to be anodic with respect to the electroconductive layer, electrons injected from the electroconductive layer to the strong field drift layer drift through the strong field drift layer, and are emitted through the surface electrodes; wherein a current restraining member for restraining a current that does not contribute for emission of a current flowing through the drift portion is provided in at least one of the electroconductive layer, the surface electrode, a portion between the electroconductive layer and the drift portions, and a portion between the surface electrode and the drift portion.
According to a first aspect of the present invention, the current restraining member is a leakage-current preventing member for preventing a current from leaking into the surface electrode from the electroconductive layer, thereby reducing the amount of power consumption. In this case, a leakage-current flow can be prevented, and a reduction in power consumption can be implemented without reducing the per-unit-area field emission area in comparison to the conventional cases.
In the electron source, the leakage-current preventing member is preferably a semiconductor layer including a pn junction. In this case, the pn junction is used to enable the leakage-current flow to be prevented.
The leakage-current preventing member may be a semiconductor layer including an n-layer on the side of the electroconductive layer and a p-layer on the side of the surface electrode. In this case, a rectification property of a pn junction of the semiconductor layer including the n-layer and the p-layer to enable the leakage-current flow to be prevented.
In a case where the leakage-current preventing member is the semiconductor layer including the n-layer on the side of the electroconductive layer and the p-layer on the side of the surface electrode, a low-concentration semiconductor layer may be formed between the p-layer and the drift portion. In this case, a rectification property of a pn junction of the semiconductor layer including the n-layer and the p-layer to enable the leakage-current flow to be prevented. In addition, according to the low-concentration semiconductor layer, the semiconductor layer including the n-layer and the p-layer and drift portion can be spatially isolated, and the drift portion can be formed without being influenced by the semiconductor layer.
In the electron source, in a case where the substrate is a semiconductor substrate, the electroconductive layer preferably includes an n-layer on the side of the substrate and a p-layer on the side of the surface electrode. In this case, since the electroconductive layer can formed using an ordinary silicon process, and the patter precision of the electroconductive layer can be improved, the display precision can be easily improved.
An i-layer may be provided between the p-layer and the n-layer. In this case, in comparison to the case where the rectification property of the pn junction is used to prevent the leakage-current flow, improvement in resistance can be implemented.
In the electron source, the surface electrode is preferably formed of a material that is to be coupled with a Schottky junction to the drift portion. In this case, a rectification property of the Schottky junction is used to enable the leakage-current flow to be prevented. Moreover, since junctions such as a pn junction and a pin junction need not be additionally provided, the structure of the electron source is simplified.
In the electron source, in a case where a low-concentration semiconductor layer is provided between the electroconductive layer and the drift portion, the electroconductive layer is preferably formed of a material that is to be coupled with a Schottky junction to the low-concentration semiconductor layer. Also in this case, a rectification property of the Schottky junction is used to enable the leakage-current flow to be prevented. Moreover, since junctions such as a pn junction and a pin junction need not be additionally provided, the structure of the electron source is simplified.
According to a second aspect of the present invention, an isolating portion for isolating the drift portions arranged adjacent to each other is provided. In addition, the current restraining member is a field moderating member for reducing the field intensity in a vicinity of a boundary to the isolating portion in the drift portion to be lower than the field intensity in a central portion of the drift portion to thereby reduce power consumption. In this case, since the field intensity in the vicinity of the boundary becomes lower than the field intensity in the central portion, and most of electrons drifting through the drift portion are led pass through the central portion, excessive electrons can be prevented from being emitted. Moreover, since the field intensity in the vicinity of the boundary becomes lower than the field intensity in the central portion, dielectric breakdown in the vicinity of the boundary can be prevented, and an overcurrent can be prevented from locally flowing between the electroconductive layer and the surface electrode.
In the electron source, the field moderating member may be an insulator film interposed between the drift portion and the surface electrode in a position corresponding to the vicinity of the boundary. In this case, when a matrix structure is employed in which a plurality of the surface electrodes and a plurality of the electroconductive layers are arranged in directions intersecting with each other, portions between individual pairs of the adjacent surface electrodes can be insulated by the insulator films.
The field moderating member may be an insulator film disposed on the electroconductive layer in a position corresponding to the vicinity of the boundary. In this case, when a matrix structure is employed in which a plurality of the surface electrodes and a plurality of the electroconductive layers are arranged in directions intersecting with each other, occurrence of crosstalk can be prevented.
The field moderating member may be formed of a high resistance layer in a position corresponding to the vicinity of the boundary, and a low resistance layer interposed between the drift portion and the electroconductive layer in a position corresponding to a central portion of the drift portion. In this case, pattern restrictions can be eliminated for the surface electrode and the electroconductive layer.
The field moderating member may be a cutout portion formed in the surface electrode in a position corresponding to the vicinity of the boundary. In this case, excessive electrons can be prevented from being emitted only by changing the pattern of the surface electrode.
The field moderating member may be a cutout portion formed in the electroconductive layer in a position corresponding to the vicinity of the boundary. In this case, excessive electrons can be prevented from being emitted only by changing the pattern of the electroconductive layer.
According to a third aspect of the present invention, the current restraining member is a field moderating layer that is disposed between the drift layer and the surface electrode and that reduces the field intensity of the strong field drift layer to thereby reduce power consumption. In this case, the field intensity in a portion of the drift layer in which dielectric breakdown tends to occur can be reduced, and hence dielectric breakdown in that portion can be prevented. Consequently, the ageing stability in the electron emission properties such as electron emission efficiency can be improved; and when the above is applied to, for example, a display, a gradual reduction in the screen luminance can be prevented. With the field moderating member being provided, the field intensity to be applied between the surface electrode and the electroconductive substrate is reduced. As such, when voltage to be applied between the surface electrode and the electroconductive substrate is controlled to be the same as that in the conventional electron source not including the field moderating layer, an emitted electron current is reduced smaller than that in the case where the field moderating layer is not provided. However, by increasing the voltage, the level of the emitted electron current can be increased to become equivalent to that in the conventional case.
In the electron source, the field moderating member may be one of a silicon nitride film and a multilayer film including a silicon nitride film. In this case, since the resistivity of the silicon nitride film is high, the film thickness of the field moderating member can be reduced. In addition, since electrons drifted through the drift layer are not easily diffused in the silicon nitride film, reduction in electron emission efficiency because of the field moderating member being provided can be inhibited.
The field moderating member may be formed of a silicon nitride film and an silicon oxide film disposed on the silicon nitride film. In this case, since the resistivities of the silicon nitride film and the silicon oxide film are high, the film thickness of the field moderating member can be reduced. In addition, since electrons drifted through the drift layer are not easily diffused in the silicon nitride film, reduction in electron emission efficiency because of the field moderating member being provided can be inhibited. Moreover, in comparison to a case where the surface electrode is formed on the silicon nitride film, since the surface electrode is formed on the silicon oxide film, the electron movement to the surface electrode easily occurs, thereby enabling the electron emission efficiency to increase.
The field moderating member may be formed of a silicon oxide film, a silicon nitride film disposed on the silicon oxide film, and another silicon oxide film formed on the silicon nitride film. In this case, since the resistivities of the silicon nitride film and the silicon oxide film are high, the film thickness of the field moderating member can be reduced. In addition, since electrons drifted through the drift layer are not easily diffused in the silicon nitride film, reduction in electron emission efficiency because of the field moderating member being provided can be inhibited. Moreover, in comparison to a case where the surface electrode is formed on the silicon nitride film, since the surface electrode is formed on the other silicon oxide film, the electron movement to the surface electrode easily occurs, thereby enabling the electron emission efficiency to increase.
The field moderating member is preferably formed of a material having a high property of adhesion to the surface electrode. In this case, ageing deterioration and ageing variations in electron emission property that can occur because of separation of the surface electrode can be inhibited.
For the material having the high property of adhesion, a chrome oxide film may be used. Since the chrome oxide film has a high transmittance property, reduction in electron emission efficiency because of the field moderating member being provided can be inhibited.
A resistance value of the field moderating member is preferably on the same order of a resistance value of the strong field drift layer. In this case, in comparison to is a case where the field moderating member is not provided, the field intensity of the drift layer can be moderated without greatly increasing the voltage to be applied between the surface electrode and the electroconductive substrate.
According to a fourth aspect of the present invention, a bus electrode commonly coupling a plurality of the surface electrodes is provided. Moreover, the current restraining member is an overcurrent protection element for limiting a current flowing between the surface electrode and the bus electrode to thereby reduce power consumption. In this case, when an associated set of the bus electrode and a wiring is appropriately selected, and voltage is applied to the selected set, a strong field is applied only to the drift portion positioned below the surface electrode that is proximate to a portion corresponding to an intersection with the wiring in the selected bus electrode, and electrons are thereby emitted. As such, the electron source can be used as an electron source for a display. Moreover, an overcurrent can be prevented from continually flowing to the surface electrode, drift portion, or the wiring, increase in the temperature thereof can be inhibited. Consequently, a deterioration range can be prevented from being increased, and the reliability can be improved.
The overcurrent protection element may be a member that causes disconnection when an overcurrent flows between the surface electrode and the bus electrode. In this case, when an overcurrent flows to a specific one of the surface electrodes, disconnection is caused between the surface electrode and the bus electrode. As such, an overcurrent can be prevented from continually flowing to the specific one of the surface electrodes. Consequently, a deterioration range can be prevented from being increased because of heat generation, and the reliability can be improved.
The overcurrent protection element may be a high resistance layer disposed between the surface electrode and the bus electrode. As such, an overcurrent can be prevented from flowing to the surface electrode. Consequently, a deterioration range can be prevented from being increased because of heat generation, and the reliability can be improved.
The overcurrent protection element may be a thermo-sensitive layer that is disposed between the surface electrode and the bus electrode and that has a positive resistance temperature coefficient. When an overcurrent flows to a specific one of the surface electrodes, and the temperature increases, the resistance of the thermo-sensitive layer increases to thereby limit a current flowing to the surface electrode, a deterioration range can be prevented from being increased because of heat generation, and the reliability can be improved.
According to fifth aspect of the present invention, the current restraining member is an electron-emission restraining member for restraining electron emission from a peripheral portion of the drift portion to thereby reduce power consumption. In this case, since electron emission from a peripheral portion of the drift portion, when the electron source is adapted in a display, occurrence of bleeding can be prevented, and a high-precision display can be implemented.
The current restraining member may be a metal layer. In this case, when the thickness of the metal layer is set larger than the mean free path of electrons, electrons can be prevented from being emitted through a position positioned below the metal layer in a peripheral portion of the drift portion.
The metal layer is preferably disposed around the drift portion. In this case, when the thickness of the metal layer is set larger than the mean free path of electrons, electrons can be prevented from being emitted through the overall peripheral portion of the drift portion, and a higher-precision display can be implemented.
In a case where the electron source includes a bus electrode commonly connecting a plurality of the surface electrode, a portion of the bus electrode may be concurrently used as the metal layer. In this case, when the thickness of the bus electrode is set larger than the mean free path of electrons, with the bus electrodes, electrons can be prevented from being emitted through a peripheral portion of the drift portion. In this case, when the electron source is adapted in a display, occurrence of bleeding can be prevented, and a high-precision display can be implemented.
In a case where the electron source includes a bus electrode commonly connecting a plurality of the surface electrode, the bus electrode is preferably disposed on two sides of a pixel. In this case, when the electron source is adapted in a display, occurrence of bleeding can be prevented, and a high-precision display can be implemented.